1,564 research outputs found
Massive star evolution in close binaries:conditions for homogeneous chemical evolution
We investigate the impact of tidal interactions, before any mass transfer, on
various properties of the stellar models. We study the conditions for obtaining
homogeneous evolution triggered by tidal interactions, and for avoiding any
Roche lobe overflow during the Main-Sequence phase. We consider the case of
rotating stars computed with a strong coupling mediated by an interior magnetic
field. In models without any tidal interaction (single stars and wide
binaries), homogeneous evolution in solid body rotating models is obtained when
two conditions are realized: the initial rotation must be high enough, the loss
of angular momentum by stellar winds should be modest. This last point favors
metal-poor fast rotating stars. In models with tidal interactions, homogeneous
evolution is obtained when rotation imposed by synchronization is high enough
(typically a time-averaged surface velocities during the Main-Sequence phase
above 250 km s), whatever the mass losses. In close binaries, mixing is
stronger at higher than at lower metallicities. Homogeneous evolution is thus
favored at higher metallicities. Roche lobe overflow avoidance is favored at
lower metallicities due to the fact that stars with less metals remain more
compact. We study also the impact of different processes for the angular
momentum transport on the surface abundances and velocities in single and close
binaries. In models where strong internal coupling is assumed, strong surface
enrichments are always associated to high surface velocities in binary or
single star models. In contrast, models computed with mild coupling may produce
strong surface enrichments associated to low surface velocities. Close binary
models may be of interest for explaining homogeneous massive stars, fast
rotating Wolf-Rayet stars, and progenitors of long soft gamma ray bursts, even
at high metallicities.Comment: 21 pages, 13 figures, 3 tables, accepted for publication in Astronomy
and Astrophysic
The Truth in Compatibilism and the truth of Libertarianism
The paper offers the outlines of a response to the often-made suggestion is that it is impossible to see how indeterminism could possibly provide us with anything that we might want in the way of freedom, anything that could really amount to control, as opposed merely to an openness in the flow of reality that would constitute merely the injection of chance, or randomness, into the unfolding of the processes which underlie our activity. It is suggested that the best first move for the libertarian is to make a number of important concessions to the compatibilist. It should be conceded, in particular, that certain sorts of alternative possibilities are neither truly available to real, worldly agents, nor required in order that those agents should act freely; and it should be admitted also that it is the compatibilist who tends to give the most plausible sorts of analyses of many of the ‘can’ and ‘could have’ statements which seem to need to be assertible of those agents we regard as free. But these concessions do not bring compatibilism itself in their wake. The most promising version of libertarianism, it is argued, should be based on the idea that agency itself (and not merely some special instances of it which we might designate with the honorific appellation ‘free’) is inconsistent with determinism. This version of libertarianism, it is claimed, can avoid the objection that indeterminism is as difficult to square with true agential control as determinism can sometimes seem to be
Close binary evolution. III. Impact of tides, wind magnetic braking, and internal angular momentum transport
Massive stars with solar metallicity lose important amounts of rotational
angular momentum through their winds. When a magnetic field is present at the
surface of a star, efficient angular momentum losses can still be achieved even
when the mass-loss rate is very modest, at lower metallicities, or for
lower-initial-mass stars. In a close binary system, the effect of wind magnetic
braking also interacts with the influence of tides, resulting in a complex
evolution of rotation. We study the interactions between the process of wind
magnetic braking and tides in close binary systems. We discuss the evolution of
a 10 M star in a close binary system with a 7 M companion using
the Geneva stellar evolution code. The initial orbital period is 1.2 days. The
10 M star has a surface magnetic field of 1 kG. Various initial
rotations are considered. We use two different approaches for the internal
angular momentum transport. In one of them, angular momentum is transported by
shear and meridional currents. In the other, a strong internal magnetic field
imposes nearly perfect solid-body rotation. The evolution of the primary is
computed until the first mass-transfer episode occurs. The cases of different
values for the magnetic fields and for various orbital periods and mass ratios
are briefly discussed. We show that, independently of the initial rotation rate
of the primary and the efficiency of the internal angular momentum transport,
the surface rotation of the primary will converge, in a time that is short with
respect to the main-sequence lifetime, towards a slowly evolving velocity that
is different from the synchronization velocity. (abridged).Comment: 11 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Can variability in the effect of opioids on refractory breathlessness be explained by genetic factors?
© 2015, BMJ Publishing Group. All rights reserved. Objectives: Opioids modulate the perception of breathlessness with a considerable variation in response, with poor correlation between the required opioid dose and symptom severity. The objective of this hypothesis-generating, secondary analysis was to identify candidate single nucleotide polymorphisms (SNP) from those associated with opioid receptors, signalling or pain modulation to identify any related to intensity of breathlessness while on opioids. This can help to inform prospective studies and potentially lead to better tailoring of opioid therapy for refractory breathlessness. Setting: 17 hospice/palliative care services (tertiary services) in 11 European countries. Participants: 2294 people over 18 years of age on regular opioids for pain related to cancer or its treatment. Primary outcome measures: The relationship between morphine dose, breathlessness intensity (European Organisation for Research and Treatment of Cancer Core Quality of Life Questionnaire; EORTCQLQC30 question 8) and 112 candidate SNPs from 25 genes (n=588). Secondary outcome measures: The same measures for people on oxycodone (n=402) or fentanyl (n=429). Results: SNPs not in Hardy-Weinberg equilibrium or with allele frequencies (<5%) were removed. Univariate associations between each SNP and breathlessness intensity were determined with Benjamini-Hochberg false discovery rate set at 20%. Multivariable ordinal logistic regression, clustering over country and adjusting for available confounders, was conducted with remaining SNPs. For univariate morphine associations, 1 variant on the 5-hydroxytryptamine type 3B (HTR3B) gene, and 4 on the β-2-arrestin gene (ARRB2) were associated with more intense breathlessness. 1 SNP remained significant in the multivariable model: people with rs7103572 SNP (HTR3B gene; present in 8.4% of the population) were three times more likely to have more intense breathlessness (OR 2.86; 95% CIs 1.46 to 5.62; p=0.002). No associations were seen with fentanyl nor with oxycodone. Conclusions: This large, exploratory study identified 1 biologically plausible SNP that warrants further study in the response of breathlessness to morphine therapy
Resolving Vega and the inclination controversy with CHARA/MIRC
Optical and infrared interferometers definitively established that the
photometric standard Vega (alpha Lyrae) is a rapidly rotating star viewed
nearly pole-on. Recent independent spectroscopic analyses could not reconcile
the inferred inclination angle with the observed line profiles, preferring a
larger inclination. In order to resolve this controversy, we observed Vega
using the six-beam Michigan Infrared Combiner on the Center for High Angular
Resolution Astronomy Array. With our greater angular resolution and dense
(u,v)-coverage, we find Vega is rotating less rapidly and with a smaller
gravity darkening coefficient than previous interferometric results. Our models
are compatible with low photospheric macroturbulence and also consistent with
the possible rotational period of ~0.71 days recently reported based on
magnetic field observations. Our updated evolutionary analysis explicitly
incorporates rapid rotation, finding Vega to have a mass of 2.15+0.10_-0.15
Msun and an age 700-75+150 Myrs, substantially older than previous estimates
with errors dominated by lingering metallicity uncertainties
(Z=0.006+0.003-0.002).Comment: Accepted for publication in ApJ Letter
Momentum transfer using chirped standing wave fields: Bragg scattering
We consider momentum transfer using frequency-chirped standing wave fields.
Novel atom-beam splitter and mirror schemes based on Bragg scattering are
presented. It is shown that a predetermined number of photon momenta can be
transferred to the atoms in a single interaction zone.Comment: 4 pages, 3 figure
- …